A polymer electrolyte fuel cell is a battery that generates electricity and heat simultaneously by allowing a fuel gas such as hydrogen and an oxidant gas such as air, which are reaction gases, to electrochemically react respectively with an anode and a cathode, which are gas diffusion electrodes. FIG. 8 shows a cross sectional view of a relevant part of a typical type of a basic component (unit cell) of such a polymer electrolyte fuel cell. As shown in FIG. 8, a fuel cell 100 includes at least one unit cell composed mainly of a membrane electrode assembly (MEA) 110, and a pair of separator plates sandwiching the membrane electrode assembly 110, specifically, an anode-side separator 120 and a cathode-side separator 130.
The membrane electrode assembly 110 has a constitution in which a polymer electrolyte membrane 111 that selectively transports cations (hydrogen ions) is disposed between the anode 112 and the cathode 113. Further, the anode 112 includes at least a catalyst layer 114 disposed in tight contact with the polymer electrolyte membrane 111 and a gas diffusion layer 116 disposed between the catalyst layer 114 and the anode-side separator 120. The cathode 113 includes at least a catalyst layer 115 disposed in tight contact with the polymer electrolyte membrane 111 and a gas diffusion layer 117 disposed between the catalyst layer 115 and the cathode-side separator 130.
The catalyst layers 114 and 115 are layers composed mainly of a conductive carbon powder carrying an electrode catalyst (e.g., a platinum group metal). The gas diffusion layers 116 and 117 are layers having gas permeability and conductivity. The gas diffusion layers 116 and 117 are obtained by, for example, forming a conductive water repellent layer composed of a conductive carbon powder and a fluorocarbon resin on a conductive porous substrate made of carbon.
As shown in FIG. 8, in the MEA 110, for the purpose of disposing sealing members (gaskets) 140 and 150 for preventing gas leakage, the polymer electrolyte membrane 111 is configured such that the main surface of the polymer electrolyte membrane 111 is larger in size than the main surfaces of the anode 112 and the cathode 113 and that the entire outer periphery of the polymer electrolyte membrane 111 extends outward from the outer peripheries of the anode 112 and the cathode 113. As used herein, the outer periphery of the polymer electrolyte membrane 111 that extends outward from the outer peripheries of the anode 112 and the cathode 113 is also referred to as “extended portion” (the portion indicated by “P” in FIG. 8).
The anode-side separator 120 and the cathode-side separator 130 have conductivity and serve to mechanically fix the MEA 110 and to electrically connect adjacent MEAs 110 in series when a plurality of MEAs 110 are stacked. In the anode-side separator 120 and the cathode-side separator 130, a gas channel 124, 135 for supplying a reaction gas to the anode 120 or the cathode 130 and discharging a gas containing a product produced by an electrode reaction and an unreacted reactant to the outside of the MEA 110 is formed in one surface (i.e., the main surfaces of the anode-side separator 120 and the cathode-side separator 130 to be in contact with the anode 112 and the cathode 113, respectively).
Further, on the other surfaces of the anode-side separator 120 and the cathode-side separator 130, a cooling fluid channel 127, 137 for introducing a cooling fluid (cooling water, etc.) to adjust the cell temperature to be almost constant is formed. With the constitution that allows a cooling fluid to circulate between the fuel cell and a heat exchanger disposed outside the fuel cell, it is possible to utilize a thermal energy generated by the reaction in the form of hot water or the like.
To simplify the production process, the gas channels 124 and 135 are usually provided by a method in which a groove is formed on the main surfaces of the anode-side separator 120 and the cathode-side separator 130 to be in contact with the anode 112 and the cathode 113. The cooling fluid channels 127 and 137 are usually provided by a method in which a groove is formed on the main surfaces of the anode-side separator 120 and the cathode-side separator 130 facing the outside.
In a so-called stack type fuel cell (fuel cell stack) obtained by stacking a plurality of MEAs 110 with anode-side separators 120 and cathode-side separators 130 interposed between the plurality of MEAs 110 and electrically connecting them in series, a manifold is provided for branching a reaction gas supplied to the fuel cell to supply the gas to each MEA 110 (a manifold formed by combining manifold apertures for supplying a reaction gas and manifold apertures for discharging a reaction gas formed in the anode-side separators 120 and the cathode-side separators 130 which are successively stacked (not shown)).
Another manifold is provided for branching a cooling fluid supplied to the fuel cell to supply the fluid to each MEA 110 (a manifold formed by combining manifold apertures for supplying a cooling fluid and manifold apertures for discharging a cooling fluid formed in the anode-side separators 120 and the cathode-side separators 130 which are successively stacked (not shown)). The manifolds formed inside the fuel cell as described above are called “internal manifolds”, and the “internal manifold type” fuel cells are commonly employed.
In the fuel cell 100, in order to prevent gas leakage of reaction gas (leakage of fuel gas to the cathode 112 side, leakage of oxidant gas to the anode 113 side, leakage of reaction gases to the outside of the MEA 110, and the like), between the anode-side separator 120 and the cathode-side separator 130 facing each other, a pair of facing sealing members having a gas sealing function, specifically, an anode-side sealing member 140 and a cathode-side sealing member 150, are disposed in the outer periphery of the MEA 110 (the outer periphery of the polymer electrolyte membrane 111 that is outside the anode 112 and the cathode 113).
In order to provide gas sealing while the anode-side sealing member 140 and the cathode-side sealing member 150 are brought into contact with the anode-side separator 120 and the anode 112, and with the cathode-side separator 130 and the cathode 113, respectively, a high dimensional accuracy, a sufficient elasticity and margin for clamping are required. For this reason, for example, O-rings made of resin and rubber, sheet-shaped sealing members, composite sheets composed of an elastic resin and a rigid resin, and the like are used. From the view point of handling of MEA 110, usually, a sealing member made of a composite material having a certain rigidity is combined with the MEA 110 for use.
Recently, as disclosed in Patent Documents 1 and 2, attempts are made to reduce the load of sealing member necessary for sealing so as to reduce the clamping load of fuel cell stack and to achieve weight reduction, simplification and low cost of structural members, and in addition to sealing members having an O-ring shape, sealing members whose cross section is triangular, semicircular and the like are also proposed. Further attempts are also made to provide a sealing member having a certain cross sectional area such as an O-ring to a separator plate.
By disposing the anode-side sealing member 140 and the cathode-side sealing member 150 such that they sandwich the entire extended portion of the polymer electrolyte membrane 111 described above, one enclosed space that encases the anode 112 is formed by the anode-side separator 120, the polymer electrolyte membrane 111 and the anode-side sealing member 140, and another enclosed space that encases the cathode 113 is formed by the cathode-side separator 130, the polymer electrolyte membrane 111 and the cathode-side sealing member 150.
These enclosed spaces serve to prevent gas leakage of reaction gases supplied to the anode 112 and the cathode 113. The combination of the anode-side sealing member 140 and the cathode-side sealing member 150 with the membrane electrode assembly 110 is sometimes called “membrane electrode sealing assembly (MESA)”.
When disposing the anode-side sealing member 140 and the cathode-side sealing member 150 in the above-described position, machining tolerances of components and assembly tolerances always occur. For this reason, it is extremely difficult to bring the anode-side sealing member 140 and the cathode-side sealing member 150 into sufficient contact with the end faces of the anode 112 and the cathode 113, respectively. Accordingly, as shown in FIG. 8, when disposing the anode-side sealing member 140 and the cathode-side sealing member 150 in the above-described position, a gap is likely to be formed between the anode-side sealing member 140 and the anode 112 and between the cathode-side sealing member 150 and the cathode 113 (i.e., an anode-side gap 112a and a cathode-side gap 113a).
If the anode-side gap 112a and the cathode-side gap 113a as described above are formed, reaction gases may leak into the anode-side gap 112a and the cathode-side gap 113a. Also, there is a problem that part of reaction gases flows through the anode-side gap 112a and the cathode-side gap 113a to the outside of the MEA 110 without flowing through the anode 112 and the cathode 113, which makes it extremely difficult to maintain efficient power generation performance.
If the anode-side gap 112a and the cathode-side gap 113a are small, a part of the anode 112 and the cathode 113 gets into the anode-side sealing member 140 and the cathode-side sealing member 150, causing insufficient sealing, or the anode-side sealing member 140 and the cathode-side sealing member 150 contact the anode 112 and the cathode 113, deteriorating the cell performance, or an excessive surface pressure acts on the anode 112 and the cathode 113, causing damage to the polymer electrolyte membrane 111 and durability deterioration. Because of this, the anode-side gap 112a and the cathode-side gap 113a having a certain size must be formed between the anode-side sealing member 140 and the MEA 110 and between the cathode-side sealing member 150 and the MEA 110, respectively.
In contrast to the above, for example, Patent Document 1 proposes to use a tube-shaped sealant for the purpose of enlarging the anode-side gap 112a and the cathode-side gap 113a in the thickness direction of the MEA 110 while reducing the clamping force and of simplifying the processing operation while effectively ensuring desired cell performance without gas leakage.    Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 11-233128    Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-141082